A. Field of the Invention
The present invention is directed generally to the field of economizers for spring-biased DC actuators. In particular, the present invention provides a control circuit for power reduction using an electronic chopper incorporating an oscillator having a variable duty cycle responsive to the source voltage to maintain average coil holding current at a level to assure minimum required magnetomotive force (MMF), defined as units of coil turns multiplied by coil amperes, for the actuator coil and including current sensing to provide inrush current limitation lower energy operation. Additionally, the invention includes an electronic dropout switch providing high impedance for rapid current decay upon deactivation of the actuator.
B. Prior Art
Spring-biased DC actuators are employed for numerous applications including high power contactors requiring fail-safe capability. Such actuators typically require significant MMF to overcome armature and contactor inertia and spring force for closing the contactor. However, once closed, the contactor may be retained in a closed position at significantly lower MMF. Various prior art methods have been applied to provide initial high currents to the actuator coil to obtain contact closure with subsequent reduction of the current to maintain MMF at a level required to resist the spring force and maintain the contact in the closed position.
To assure minimum MMF in conditions of high temperature, wherein coil resistance may be significantly altered, and power changes due to voltage sagging in the power supply, most designs require a significant safety factor to be applied to the current level maintained in the coil, precluding optimizing of performance. Prior art systems employing chopper circuits to reduce average current in the coil must provide sufficient MMF to accommodate worst-case conditions. Higher currents which need to be maintained exacerbate the situation by contributing to heating of the coil.
In addition, the use of most prior art choppers as an economizer circuit results in difficulties during de-actuation of the circuit. Addition of a chopper controller normally increases dropout time due to slow decay of coil current through the low-impedance return circuit employed in chopper economizers. Regeneration in the coil created by motion of the armature while current is still flowing in the coil impacts the velocity and kinetic energy of the armature, thereby extending the dropout time and weakening the action that breaks open sticky contacts. The present invention overcomes the difficulties in the prior art to allow high efficiency in conservation of coil power, enhancement of mechanical performance, and potential reduction in size of actuators for given applications.